As a system for transmitting a digital signal, a modulation system called an orthogonal frequency division multiplexing (OFDM) system (hereinafter, referred to as an OFDM system) is used. In the OFDM system, a large number of orthogonal sub-carrier waves (subcarriers) are provided in the transmission band, and digital modulation is carried out by allocating data to the amplitudes and phases of the respective subcarriers by PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation).
The OFDM system has a characteristic that the total transmission rate thereof is the same as that of conventional modulation systems, although the band per one subcarrier is narrow and the modulation rate is low because the transmission band is divided by a large number of subcarriers. Furthermore, in the OFDM system, the symbol rate is low because a large number of subcarriers are transmitted in parallel, which allows a short multipath time length relative to the symbol time length. Therefore, the OFDM system has a characteristic of being unsusceptible to multipath interference.
In addition, because data allocation to plural subcarriers is carried out, the OFDM system has a characteristic that a transceiving circuit can be constructed by using an IFFT (Inverse Fast Fourier Transform) operation circuit for performing an inverse Fourier transform at the time of transmission and using an FFT (Fast Fourier Transform) operation circuit for performing a Fourier transform at the time of reception.
Due to the above-described characteristics, the OFDM system is frequently applied to digital terrestrial broadcasting, which is highly susceptible to the influence of multipath interference. Examples of the standards of such digital terrestrial broadcasting for which the OFDM system is employed include DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), and ISDB-TSB (ISDB-T Sound Broadcasting).
As shown in FIG. 1, a transmission symbol of the OFDM system (hereinafter, referred to as an OFDM symbol) is composed of an effective symbol corresponding to the signal period during which IFFT operation is performed at the time of transmission and a guard interval obtained by copying the waveform of a part of the latter half of this effective symbol as it is. The guard interval is provided in the former half of the OFDM symbol. In the OFDM system, the provision of such a guard interval permits inter-symbol interference due to multipath and enhances the resistance against multipath.
Moreover, in the OFDM system, it is defined that one transmission unit called an OFDM frame is formed by collecting plural OFDM symbols described above. For example, in the ISDB-T standard, one OFDM frame is formed by 204 OFDM symbols. In the OFDM system, on the basis of this OFDM frame unit, the insertion positions of e.g. a scattered pilot (SP) signal (hereinafter, referred to as an SP signal) used to estimate the channel characteristic and a TMCC (Transmission and Multiplexing Configuration Control)/AC (Auxiliary Channel) signal including transmission parameters and so on are defined.
An arrangement pattern of the SP signal and the TMCC/AC signal in an OFDM frame employed in the ISDB-T standard is shown in FIG. 2. The SP signal is subjected to BPSK (Binary Phase Shift Keying) modulation and inserted at the rate of one subcarrier in twelve subcarriers along the subcarrier direction (frequency direction). Furthermore, the SP signal is inserted at the rate of one time per four OFDM symbols for identical subcarriers along the OFDM symbol direction (time direction). On the other hand, the TMCC/AC signal is subjected to differential BPSK modulation and inserted to predetermined plural subcarriers. Furthermore, the TMCC/AC signal is inserted to identical subcarriers along the OFDM symbol direction (time direction) for all the OFDM symbols.
Note that, hereinafter, a subcarrier to which the TMCC/AC signal is inserted will be referred to as a pilot carrier, and a subcarrier to which a normal data signal is inserted will be referred to as a data carrier.
By the way, in an OFDM reception device that receives an OFDM signal whose transmission unit is the above-described OFDM symbol, an original carrier arrangement like that shown in FIG. 3A is often shifted toward the lower-frequency direction as shown in FIG. 3B or shifted toward the higher-frequency direction as shown in FIG. 3C due to a carrier frequency offset. Note that only 17 subcarriers are shown in FIGS. 3A to 3C for simplification. Therefore, the OFDM reception device is needed to detect the carrier frequency offset amount and remove the influence of the carrier frequency offset.
Conventionally, as a method for detecting a carrier frequency offset amount with subcarrier accuracy, there is a method in which the correlation of the above-described pilot carriers between adjacent OFDM symbols is utilized. Details of this method are described in Kenichirou Hayashi et al., “Development of OFDM Key Techniques”, Technical Report of The Institute of Image Information and Television Engineers (ITE Technical Report), Vol. 23, No. 28, pp. 25 to 30, Mar., 1999. The procedure of the detection of a carrier frequency offset amount will be described below with reference to the flowchart shown in FIG. 4.
Initially, in a step S101, the phase rotation amount with respect to the one previous OFDM symbol is calculated for each of the subcarriers of an OFDM signal obtained after quadrature demodulation. In the ideal reception state, the phase rotation amounts of pilot carriers are each 0 or 180 degrees because the pilot carriers are subjected to differential BPSK modulation, and the phase rotation amounts of data carriers are random values because the data carriers are subjected to e.g. 64 QAM modulation. In order to avoid the state in which the pilot carriers have two kinds of phase rotation amounts of 0 and 180 degrees, all the phase rotation amounts are set to 0 degrees by executing squaring processing before the calculation of the phase rotation amounts.
Subsequently, in a step S102, the assumed offset amount is defined as k, and k is set to the minimum value in the search range. The assumed offset amount k means an offset by k subcarriers from the position regarded as the center by the circuit. Subsequently, in a step S103, it is determined whether or not the assumed offset amount k is the maximum value in the search range. If it is not the maximum value, the procedure sequence proceeds to a step S104. If it is the maximum value, the procedure sequence proceeds to a step S107.
In the step S104, the phase rotation amounts of the pilot carrier positions that are defined in conformity with the standard when an offset by k subcarriers is assumed are acquired. In a step S105, the respective phase rotation amounts are mapped on a circumference having a fixed radius on a complex plane and converted to rotation vectors, and all the rotation vectors are cumulatively added to each other. Subsequently, in a step S106, k is incremented by one, so that the procedure sequence returns to the step S103.
In the step S107, the absolute values of the cumulative-addition-result values are obtained and the maximum absolute value is sought. In a step S108, the assumed offset amount k when the maximum absolute value is obtained is output as the proper carrier frequency offset amount. This operation is based on the following: when the assumed offset amount k matches the proper carrier frequency offset amount, only the rotation vectors of pilot carriers are cumulatively added and thus a large absolute value is obtained; however, when the assumed offset amount k does not match the proper carrier frequency offset amount, the rotation vectors of data carriers are cumulatively added and thus a small absolute value is obtained due to canceling-out of the rotation vectors.
For example, when the carrier frequency offset amount is −2 and k=−2, the correlations of pilot carriers between OFDM symbols are obtained as shown in FIG. 5A, and therefore the absolute value of the cumulative-addition-result value is large. In contrast, when the carrier frequency offset amount is −2 but k=+1, the correlations of data carriers between OFDM symbols are obtained as shown in FIG. 5B, and therefore the absolute value of the cumulative-addition-result value is small.